The video signal format in computers is not directly compatible with the video signal format in televisions. Thus, it is not possible to directly relay computer video signals to a television. It is advantageous to convert computer video signals into signals that may be used by a television. For example, the conversion of computer video signals into television video signals is useful in displaying computer data on a television or for recording it to a video cassette recorder (VCR). This capability is advantageous, for example, for live presentations where both the presenter and the audience need to see the computer data, to play video games on a television, or similar contexts in which it is desirable for a computer to exploit a relatively inexpensive television screen.
The conversion from computer video signals to television video signals entails the conversion of analog RGB signals to composite video signals. This operation requires a video encoder to convert computer image or graphics data (in RGB, YCbCR, or color indexed form) into a standard analog baseband television (NTSC or PAL) signal or S-VHS signal with a modulated color subcarrier.
Techniques to convert a computer video signal to a television signal are known in the art. FIG. 1 illustrates one conversion process. A computer 20 generates R,G,B signals, a horizontal synchronization signal, and a vertical synchronization signal. An interlace processing device 22 is then used to convert the non-interlaced computer signal into an interlaced signal. The interlaced signal is then stored in a memory 24. Two types of memory may be used. In a slave mode memory, only a few lines of a video frame are stored. In a master mode memory, an entire video frame is stored. A master mode memory device has much higher memory requirements and therefore is more expensive than a slave mode memory device.
The stored data from the memory 24 is relayed to an analog encoder 26. The analog encoder 26 processes the data to generate a video signal for a television 32. The processing in the analog encoder 26 relies upon a pixel clock that is generated by a phase-locked loop (PLL) 28, whose input is the horizontal synchronization signal from the computer 20. The analog encoder 26 also receives a frequency subcarrier signal (F.sub.-- sc) from an oscillator 30.
In an analog encoder 26, a separate oscillator 30 may be set directly to the required subcarrier frequency. This option is not available for a digital encoder. A digital encoder is advantageous because it is more accurate than an analog encoder. Unfortunately, a digital encoder is difficult to implement. The difficulty arises from the fact that a single system clock must be used in a digital encoder. Thus, the subcarrier frequency must be derived from the clock signal from the computer 20. In other words, a separate oscillator 30 set to the subcarrier frequency cannot be exploited.
The problem with relying upon the clock signal from the computer 20 is that it is relatively inaccurate in terms of subcarrier frequency. As shown in FIG. 1, an encoder relies upon the pixel clock and the color subcarrier frequency (F.sub.-- sc). Frame rate, field rate, vertical and horizontal timing are all directly derived from the pixel clock, while the color information is carried by the color subcarrier signal. A television imposes different accuracy requirements for these two signals. Errors in the pixel clock cause horizontal and vertical timing errors, but most televisions and video cassette recorders can tolerate the errors. On the other hand, for most televisions and video cassette recorders, the color subcarrier signal must be accurate to within about .+-.200 Hz. Television standards stipulate that the color subcarrier frequency be accurate to within .+-.10 Hz.
Since a master mode memory stores an entire video frame, it operates as a time buffer between the computer domain and the television domain. The timing circuitry required to store a video frame in the master mode memory can be used to generate an accurate color subcarrier frequency signal. Thus, a digital encoder can be conveniently used in a master mode memory device.
On the other hand, in a slave mode memory device, data is passed through the memory device on the fly. Thus, there is no time buffer between the computer domain and the television domain. Instead, the transition from the computer domain to the television domain must be accomplished by deriving the color subcarrier frequency signal from the horizontal synchronization signal. Specifically, the color subcarrier frequency signal must be derived by initially multiplying the horizontal synchronization signal by a constant to obtain the pixel clock. The pixel clock must then be used to generate the color subcarrier frequency signal.
Since the color subcarrier frequency signal is derived from the horizontal synchronization signal, it is important to have an accurate horizontal synchronization signal. Unfortunately, the horizontal synchronization signal from a computer is not accurate, it usually varies (from board to board) up to .+-.1%. The inaccuracy in the horizontal synchronization signal can result in an inaccurate color subcarrier frequency signal that precludes a television from reproducing the required color information.
Thus, it would be highly desirable to accurately derive a television color subcarrier frequency signal from an imprecise horizontal synchronization signal. This would enable a slave mode device, with inexpensive memory, to use a digital encoder to convert computer video signals into color television signals. The digital encoder would thereby provide a highly accurate reproduction of computer video signals for display on a standard television.